#yi {
    width: 200px;
    height: 100px;
    background-color: brown;
    text-align: center;
    line-height: 100px;
}

#er {
    width: 200px;
    height: 300px;
    background-color: blue;
}

#box {
    width: 50px;
    height: 50px;
    position: absolute;
    background: red;
}